Read books to her at her intellectual level, and/or try audio books.
Read a page of her school reader, then let her read the next page to you. Or even, if it's a really bad night, read a page, and let her read it back to you.
While understanding of phonics is the most important part of learning to read, actual enjoyment of reading is also very important. Sounding out every second word will slow down the flow of the story so as to make it meaningless. It becomes a chore, not something to enjoy.
If she struggles on a word, say it to her, and let her move on. Then, go back to it at the end of the book, or session, for practise at sounding it out.
DS was always being given books he hated from school. I started buying Usborne Beginners for him, which are small non fiction books, about almost every subject you can think of. (We initially got some from the library.) He amazed me at his ability to read about something he was interested in.
He difficulties with reading may well be what's put her in lower ability for everything. How can you do a maths sheet if you can't read it? And obviously humanities subjects are all about reading and writing. Ditto much of science, at this stage. Even in art and DT, they have to write about method etc.
Is her verbal ability far in advance of her written ability? Perhaps have her tested by an Ed Psych. If she is dyslexic, it could be helpful for her to understand that she has a physiological problem (in the way that poor eyesight is a physiological problem) and that she's not actually unintelligent (which is probably how she feels right now, with her friends zooming past her).